Try Firefox Safe Mode to see if the problem goes away. Firefox Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes).

If Firefox is open, you can restart in Firefox Safe Mode from the Help menu:

• Click the menu button , click Help and select Restart with Add-ons Disabled.

If Firefox is not running, you can start Firefox in Safe Mode as follows:

• On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac: Hold the option key while starting Firefox.
• On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode
  (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)

When the Firefox Safe Mode window appears, select "Start in Safe Mode".

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, theme, or hardware acceleration. Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause.

To exit Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help others with the same problem.

Another thing to look at is the optional userChrome.css file if you are using one. The code for the Tab Bar was changes a bit in Firefox 66 and has caused an issue for a few users who posted here in the last week.

the-edmeister said

Try Firefox Safe Mode to see if the problem goes away. Firefox Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes). If Firefox is open, you can restart in Firefox Safe Mode from the Help menu:

• Click the menu button , click Help and select Restart with Add-ons Disabled.

If Firefox is not running, you can start Firefox in Safe Mode as follows:

• On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac: Hold the option key while starting Firefox.
• On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode
  (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)

When the Firefox Safe Mode window appears, select "Start in Safe Mode".

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, theme, or hardware acceleration. Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause. To exit Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again. When you figure out what's causing your issues, please let us know. It might help others with the same problem. Another thing to look at is the optional userChrome.css file if you are using one. The code for the Tab Bar was changes a bit in Firefox 66 and has caused an issue for a few users who posted here in the last week.

Hi, thanks for the answer.

Starting in safe mode the bar works. I tried to disable all extensions and disable hardware acceleration as said in the troubleshoot, and started again with the same problem

What about userChrome.css?

If you are using a userChrome.css file, with Firefox 66 that is the most likely culprit. The code for "tabs" was changed for 66 and the user would need to fix that code for compatibility going forward from Fx65.

the-edmeister said

What about userChrome.css? If you are using a userChrome.css file, with Firefox 66 that is the most likely culprit. The code for "tabs" was changed for 66 and the user would need to fix that code for compatibility going forward from Fx65.

Worked. Thanks a lot. I didn't created that file (that's why I didn't even checked if it was the problem), don't know how it was there

How can i do that?

lompos said

How can i do that?

Go to your profile folder and find a chrome folder, the file will be there. http://kb.mozillazine.org/Profile_folder